Biosynthesis of type I procollagen. Characterization of the distribution of chain sizes and extent of hydroxylation of polysome-associated pro-alpha-chains.

[3H]Proline-labeled nascent procollagen chains were isolated from chick tendon polysome preparations as peptidyl-tRNA complexes by ion exchange chromatography. Proline hydroxylation of the nascent chains was at least 40% complete, based on radioactive hydroxyproline/proline ratios. These data provide the first direct evidence that hydroxylation of procollagen proline residues does occur on nascent chains. The electrophoretic profiles of [3H]proline-labeled nascent chains and of unlabeled nascent chains visualized by Western blotting with 35S-labeled monoclonal antibodies to the alpha 1(I) N-propeptide or the C-propeptides indicate that there are pauses in the translation of procollagen alpha-chains in the intact cells. Approximately 25% of the radioactivity associated with [3H]proline-labeled polysomes was in fully elongated but underhydroxylated (relative to secreted procollagen) pro-alpha-chains. The association of these completely elongated but only partially modified procollagen chains with the polysome complex may facilitate the carboxyl-terminal interactions which lead to triple helix formation.